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Starting
with Garland
You may
have seen the images by Mark Hamilton,
with nature paintings of mountains and trees. For a number of pine
trees, he used particle brushes which start with the Garland preset and
modified slightly to change the look and effect.

Here is a summary of the most important parameters used:

starting with preset: Garland

Max Particles > 50 (!)

Particles Emission > 10

Mass > 0.5

Gravity > -0.1

Initial > 1

Terminal Velocity > 9

Drag > 0

Life Span > 4

Spits > 4

Spit Angle > 15

Randomize Values > 0.3

Style > Line

Check Tinted

Let's take a closer look at what happens when adjusting some of these
parameters.

Start from the list of particle presets (Settings).

Select the Garland preset

Draw a single brush stroke from left to right.

Do another one but use the SHIFT key to end it. When you press and hold
the Shift key, current particles continue until they end, but no new
ones are generated.

Using the SHIFT key to end the particles can make a cleaner ending
especially if there are big contrasts between start and end of the
color gradient used. You don't usually want to see a stub at the end
showing the unfinished start of a new set of particles that didn't get
a chance to live their lifespan.

The Max
Particles factor

The original settings have a max particles count of several hundreds.
Change it to 50

After reducing the count to a max of 50, draw a stroke from left to
right, at various speeds.

Notice how the
needles seem to grow longer if you draw faster. This is
due to one of the parameters that sets the dependency on the speed of
the mouse (mouse velocity)

Initial
Velocity

Another crucial parameter is the initial velocity of the particles. Try
a few values: 1, 2, 3,...

When the initial
velocity is higher, the particles immediately shoot out farther away
from their originating point, and create longer needles. You can use
this also to mimic the effect of perspective, with shorter needles used
for trees which are farther away.

The Drag
Factor

The Drag factor is sort of a friction effect which slows
down particles as if they were placed not in thin air or a vacumm, but
in thick fluid.

Or with a negative value they overshoot into the opposite direction of
your mouse motion. So, while you may be drawing to the right, the needs
may get a left-ward slant. One of the better known presets that uses
such contrarian setting is the 'Spray branches' reset. If you use a
high value in the other direction, it sort of accelerates into your
direction, leaping ahead of the mouse.

Try a few negative and positive values: Note that using 1 doesn't
produce any particles, as they stay still. (not shown here)

The
Relationship between Lifespan and Splits

There are several ways to make longer particles. You can for example
use mouse velocity and draw faster strokes, which will make the
particles move faster. You can also work with the drag parameter.

The most common way is simply to give them a longer lifespan.

However,
there's also another detail: the number of Splits. Each time that
a particle splits, it runs through anout full measure of the lifespan.
In other words, the Lifespan is not the overall full duration of the
particle's life, it is how long it lives just for each split. Thus if
you increase the Split count, it makes for longer resulting trails too.

When you use the
Split parameter, it can however add a twist to it: literally. If the
Split angle is not zero, it will gradually turn the subsequent split
trails on and on, resulting in a turning path.

Try a few combinations of Lifespans and splits which multiply to 32:
Start with a 32 Lifespan and 1 split, then decrease the lifespan (16,
8, 4,...) and increase the splits at the same time:

Lifespan

32

16

8

4

2

1

Splits

1

2

4

8

16

32

This is the result of long lifespans and low splits

As you decrease the lifespan and increase the number of splits, you
notice a gradualy turning (counterclockwise) of the needs.

You could of course keep the Lifespan longer, independently of
the Splits count.

Soon it looks like some italian pasta like balls of angel hair ;-)

Max
Particles & Particles per Emission

Another parameter to explore some more is the Particles per emission,
and how it relates to the Max particles count.

When
you first draw with particles, say with a Max count set at 555 and
count per emissions of 44 as set here, it generates a burst of 44
particles all at once.
It will take a while for these particles to finish painting, especially
if there's a long lifespan and split count for them.

If the 'Randomize Value' parameter is very small, then the particles
will all reach the end of their life at around the same time, and
another batch of 44 particles will be spawned off. This creates another
'blobb' of particle emission, shown below the first in the left-most
column below. Then, if you reduce the particles per emission to 22, or
15, you notice that the blobs stretch further down: fewer particles are
launched each time, and a few of them reach their end early enough that
a few new ones can spawn off until the bult of the others end and spawn
of in a new blob. Plus, the drawing takes less time so if the particles
follow the mouse velocity as well, they'll follow down the path better.
All in all, with even fewer particles per emision and still a little
randomize value, you'll see a constant stream of particles along the
path.

And now the fun part: play with different values for all these
parameters, such as fewer particles overall (Max), and also with
different Style, especially the Brush style to use the 'normal' brush
settings along the particle paths trails. For example, use one of the
smear modes or set the Mode to PaintSmear, and see the smearing occur
not only just where the mouse is painting, but over dozens of particle
paths. It's like having dozens of busy hands applying the smearing
effects of the normal brush, along all particle trails

There's much more fun to be discovered. Here's something easily done
with additional post work (COlor Solbel Edge detect and Light
Diffusion). Now you can create new life forms for deep sea ocean
illustrations!

Here's another example, of PD Pro at work to make a tree with bushels
of pine needles: